The 2015 z06 should HANDILLY beat the GTR in the hands of any good driver on nearly any track on any day. Heck, the z06 should even beat the NISMO GTR($150,000) in most cases(a more curvy track will favor the corvette) with a good driver.
The base model GTR (not the black, or the Nismo) is more in line with the base corvette, and a base GTR starts at $101,000 while the base corvette is $55,000
For example at VIR, the 2015 z06 posted a 2:41.3 which is FASTER than the 918 at 2:43.1 and a 918 is a million dollars! (to be fair to the Porche, these laps were on different days with different drivers, but still)
Just look:
http://www.caranddriver.com/features/lightning-lap-2014-ll2-class-feature
The base corvette ($65,000/tested ) absolutely DOMINATED it's class in the Lightning lap, over 12 seconds faster than the next fastest.
It even beats everything in the next class higher
http://www.caranddriver.com/features/lightning-lap-2014-ll3-class-feature
except for GM's own z/28 camaro. (this is the base corvette were talking about here)
The base corvette is 8.8 seconds faster than the Porsche Cayman S;
The base corvette is even faster that the Audi R8 V-10 Plus which is almost 3 times the price
Around VIR the Nismo GTR does a 2:49.4, again, over 8 seconds slower than a z06 vette, and only 4.4 seconds faster than the base corvette
Heck, the NISMO GTR is only 1.5 seconds faster than a z/28 Camaro at VIR.
AND the base Corvette beats all of these cars above while getting 30 MPG on the highway in eco-mode.
This is the standard that a world class super car should use to measure and I see corvettes are way down at the bottom. They're not slow, but just not as fast.
https://nurburgringlaptimes.com/lap-times-top-100/